1. Field of the Invention
This invention relates to bicycle crankshaft assemblies designed to be of minimal weight and maximal strength and rigidity, and to allow ease of assembly, adjustment and disassembly.
2. Description of the Prior Art
There is much demand for light, rigid and strong crankshaft assemblies. Because the crankshaft assembly is z-shaped, and must be installed so that its center portion is located within a relatively narrow diameter hub (known as the bottom bracket shell) of the frame of the bicycle, if a crankshaft assembly has no detachable joints but is one-piece, it must be shaped in a way which yields poor strength and rigidity relative to weight.
For this reason, higher quality crankshaft assemblies utilize detachable joints: they consist of two or three parts which are removably attached to form the complete assembly. Because joints are inherently the weakest point in a structure, including a crankshaft assembly (other things being equal), the location of the joints in a crankshaft assembly is critical to achieving the optimum relation of strength and rigidity to weight.
The typical three-piece crankshaft assemblies used in higher quality bicycles utilize two joints. The end of each crank arm opposite the pedal (the "pedal-opposite end" or "spindle end") is removably attached to an end of the shaft or spindle. Unfortunately however, the junction of the crank arm and spindle is the least desirable joint location for at least four reasons:
1. The junction of the crank arm and spindle is the point of highest stress and load in the crankshaft assembly. PA1 2. To accommodate this great stress and load, the joints, including both the ends of the spindle and the pedal-opposite ends of the crank arms, require reinforcement through extra material which excessively increases the weight of the crankshaft assembly. PA1 3. The necessity of enabling the pedal-opposite end of the crank arm and the end of the spindle to be separable or detachable, and the design requirements of the joint to accommodate the great stress and load to which it is subjected, prevent both the spindle and the crank arms from being shaped in the way that best maximizes their strength and rigidity while minimizing their weight. PA1 4. Finally, at this location, the joints are especially subject to the elements and therefore prone to corrosion. PA1 a. each crank arm is integrally connected to the corresponding end of a two-piece spindle, PA1 b. the detachable joint between the two spindle portions is located within the bottom bracket between the bearings, and PA1 c. the pedal and spindle ends of the crank arms are reinforced in a manner which maximizes their strength and rigidity with minimal additional weight; PA1 a. transferring the detachable joint of the various components of the crankshaft assembly from an area of high stress to an area of lower stress, PA1 b. allowing both the spindle and the crank arms to be shaped and joined in the way that best maximizes their strength and rigidity while minimizing their weight; PA1 c. enabling the crankshaft assembly to be easily and conveniently installed and removed with one common tool; PA1 d. enabling the crankshaft assembly to be easily and conveniently adjusted to align the sprocket or sprockets with the chain line; PA1 e. leaving the bearings free from any pre-load; PA1 f. minimizing the hardware necessary to removably join the components of the crankshaft assembly; and PA1 g. enabling the junction of the various components, and the hardware for removably accomplishing this junction, to be hidden within the bottom bracket compartment, and protected from the elements.
To lessen these disadvantages, several two-piece crankshaft assemblies have recently been introduced. In these designs, the pedal-opposite end of one crank arm is permanently attached to one end of the spindle, usually by welding, while the pedal-opposite end of the other crank arm is removably attached to the other end of the spindle. Durham, U.S. Pat. No. 4,704,919 (1987) discloses one such design. These designs reduce the disadvantageous effects of the three-piece design, since on one side of the assembly the separable junction or joint between the pedal-opposite end of the crank arm and the spindle end is eliminated. However, on the other side of the assembly, the conventional separable junction location, with its disadvantages, is still utilized.
Two-piece assemblies in the early art disclosed by Ludlow and Taylor, U.S. Pat. No. 648,077 (1900); Scott, U.S. Pat. No. 627,597 (1899); Annable, U.S. Pat. No. 622,644 (1899); and Jerome, U.S. Pat. No. 623,373 (1899) avoid use of the conventional crank arm/spindle junctions. Instead, each crank arm is integrally connected to an axle or spindle portion, and the two spindle portions are removably joined. One spindle portion is solid, extends all or part way through the bottom bracket shell, and terminates in a solid, threaded stud. The other spindle portion consists of a hollow sleeve which fits over the solid spindle portion. The two spindle portions are joined by means of a nut tightened onto the threaded stud of the solid spindle portion and butting against a shoulder of the hollow spindle portion.
However, because of the first crank arm's solid axle portion with its solid threaded stud, and because of the large amount of overlap between the two axle portions, assemblies constructed according to these designs were excessively heavy and appear to be completely out of use in recent times.
Previously, neither one-piece, two-piece or three-piece crankshaft assemblies have achieved the optimum relation in the crank arms between strength and rigidity on the one hand, and lightness on the other hand. As discussed above, in three and some two-piece designs this is in part caused by choosing to conventionally detachably join the crank arm to the spindle. But even where one or both crank arms are integrally connected with the corresponding ends of the spindle, this optimum relation has not been achieved. Relatively great forces are exerted at both the pedal and the spindle ends of the crank arms, and the previous means of reinforcing these ends in order to dissipate or transfer these forces add an unnecessarily large amount of weight.
In virtually all known previous crankshaft assemblies, one-piece, two-piece and three-piece, lateral adjustment of the assembly, including the sprocket or sprockets, and sometimes also the elimination of play in the axle, are accomplished in manners which exert inward force on the outer portions of the bearings. For example, in Durham, lateral adjustment of the sprocket is accomplished through spacers or washers inserted on the spindle between the crank arms and the bearings. In conventional designs, elimination of axle play and any allowable lateral adjustment are accomplished by tightening one or both of the outer races against the bearings. As a result, in all of these assemblies the bearings are subject to pre-load. A few designs eliminate this pre-load by adding a means of exerting a compensating outward force on the inward portions of the bearings. However, this entails additional weight. The remaining assemblies all experience pre-load, causing increased resistance and wear on the bearings.
In most two and three-piece crankshaft assemblies some or all of the hardware which removably joins the various components, and in some cases this junction itself, is located outside of the bottom bracket compartment. This detracts from the clean, integrated appearance of the entire assembly, creates additional recesses and cavities which collect dirt and grime, and exposes this hardware to the elements, leaving it susceptible to corrosion and sticking or "freezing."
A final disadvantage of most two and three-piece crankshaft assemblies is that at least one, and often several, relatively expensive, specialized tools are required to install, adjust, and remove the crankshaft assembly.